1. Field of the Invention
The present invention relates generally to the production of steam from geothermal brine and especially to processes and apparatus for clarifying flashed, silica-rich geothermal brine before the reinjection thereof into the ground.
2. Discussion of the Prior Art
Large subterranean aquifers of naturally produced (geothermal) steam or hot aqueous liquids, specifically water or brine, are found throughout the world. These aquifers, which often have vast amounts of thermal energy, are most commonly found where the earth's near-surface thermal gradient is abnormally high, as evidenced by unusually great volcanic, fumarole and/or geyser activity. Thus, as an example, geothermal aquifers are fairly common along the rim of the Pacific Ocean, long known for its volcanic activity.
Geothermal steam or water has, in some regions of the world, been used for centuries for therapeutic treatment of physical infirmities and diseases. In other regions, such geothermal fluids have long been used to heat dwellings and in industrial processes. Although efforts to further develop geothermal resources for these site-restrictive uses continue, considerable recent research and development has additionally been directed to the exploitation of geothermal resources for production of electrical power, which can be conducted, often over existing power grids, for long distances from the geothermal sources. In particular, recent steep increases in the cost of petroleum products used for the conventional production of electric power, as well as actual or threatened petroleum fuel shortages and/or embargoes, have intensified the interest in use of geothermal fluids as an alternative, and generally self-renewing, source of power plant "fuel."
General processes by which geothermal fluids can be used to generate electric power are known, and have been known for some time. As an example, geothermal steam, after removal of particulate matter and such polluting gases as hydrogen sulfide and ammonia, can be used in the manner of boiler-generated steam to operate steam turbine generators.
Naturally pressurized geothermal brine or water having a temperature of over about 400.degree. F. can be flashed to a reduced pressure to convert some of the brine or water to steam. The steam produced in this manner can then be used to drive steam turbine generators. The flashed geothermal liquid and the steam condensate obtained from power generation are typically reinjected into the ground to replenish the aquifer and prevent ground subsidence. Cooler geothermal brine or water can often be used to advantage in binary systems in which a low-boiling point, secondary liquid in a closed loop is vaporized by the hot geothermal liquid, the vapor produced from the secondary liquid being used to operate gas turbine generators. The "used" brine is then typically reinjected into the ground for reasons mentioned above.
As might be expected, the use of geothermal steam is preferred over the use of geothermal water or brine for generating electric power, because the steam can be used more directly, easily and cheaply. Consequently, where readily and abundantly available, geothermal steam has been used for a number of years to generate commercially important amounts of electric power at favorable costs. For example, by the late 1970's, geothermal steam at The Geysers in Northern California was generating about two percent of all the electricity used in California.
Although energy production facilities at such important geothermal steam sources as The Geysers are generally still being expanded, the known number of important geothermal steam aquifers is small compared to that of geothermal brine or water. Current estimates are, in fact, that good geothermal brine or water sources are about five times more prevalent than are good sources of geothermal steam. The potential for generating electric power is, therefore, much greater for geothermal brine and water then it is for geothermal steam. As a result, considerable current geothermal research is understandably directed toward developing economical geothermal brine and water electric power generating plants, much of this effort being expended toward the use of vast geothermal brine resources in the Imperial Valley of Southern California.
Although, as above mentioned, general processes are known for using geothermal brine or water for producing electric power, serious problems--especially with the use of highly saline and corrosive geothermal brines--have often been encountered in actual practice. These problems have frequently been so great as to prevent the production of electric power at competitive rates and, as a consequence, have greatly impeded the progress of flashed geothermal brine power plant development in many areas.
These severe brine-handling problems are caused primarily by the typically complex chemical and corrosive nature of most geothermal brines. At natural aquifer temperatures in excess of about 400.degree. F. and pressures in the typical range of about 400 psig to about 500 psig, geothermal brines leach large amounts of salts, minerals and elements from the aquifer formation, the brines presumably being in chemical equilibrium with their producing formations. Thus, although brine composition may vary considerably from aquifer to aquifer, or even from well to well, the brines typically contain very high levels of dissolved silica, as well as substantial levels of dissolved heavy metals such as lead, copper, zinc, iron, and cadmium. In addition, many other impurities, particulate matter and dissolved gases are contained in most geothermal brines.
As natural brine pressure and temperature are substantially reduced in power plant steam production (flashing) stages, chemical equilibrium of a brine is disturbed and saturation levels of silica and other materials dissolved in the brine are commonly exceeded. This causes silica and these other materials to precipitate from the brine, as a tough, tenacious scale, onto brine-exposed equipment walls and in reinjection wells, often at a deposition rate as high as several inches a month. Assuming, as is common, that the brine is supersaturated with silica at the wellhead, in high temperature portions of the brine handling system (for example, in the high pressure brine flashing vessels) heavy metal sulfide and silicate scaling typically predominate. In lower temperature portions of the system (for example, in atmospheric flashing vessels) amorphous silica and hydrated ferric oxide scaling have been found to predominate. Scale, so formed, typically comprises iron-rich silicates, and is usually very difficult, costly and time consuming to remove from equipment. Because of the fast-growing scale rates, extensive facility down-time for descaling operations is usually required unless scale inhibiting processes are employed. Associated injection wells may also require frequent and extensive rework due to scale formation, and new injection wells may, from time to time, have to be drilled at great cost.
Therefore, considerable effort has been, and is being, directed toward developing effective processes for eliminating, or at least very substantially reducing, the formation of scale in flashed geothermal brine handling systems. One such scale-inhibiting process disclosed in U.S. Pat. No. 4,370,858 to Awerbuck, et al, involves the induced precipitation of scale-forming materials, notably silica, from the brine in the flashing stage by contacting the flashed brine with large amounts of fine silica or silica-rich seed crystals. When the amount of silica which can remain in the brine is exceeded by the brine being flashed to a reduced pressure, silica leaving solution in the brine deposits onto the seed crystals which are subsequently removed from the system. Not only do the vast number of micron-sized seed crystals introduced into the flashing stage provide a very much larger surface area for silica deposition than the exposed surfaces of the flashing vessels but also the silica from the brine tends to preferentially deposit onto the seed crystals for chemical reasons, not all of which are completely understood. As a result, with the use of such processes, most of the silica leaving the brine precipitates onto the seed crystals, instead of precipitating as scale onto vessel and equipment walls and in injection wells.
Preferably, the seed crystals used for such processes are introduced into the high pressure flashing vessel (crystallizer), wherein steam is separated from the two-phase geothermal fluid. The silica removal or crystallization process, although commencing in the high pressure flash crystallizer, continues in successive, lower-pressure flashing vessels, in which additional steam separation occurs. In a downstream reactor-clarifier, the suspended siliceous material is subsequently separated from the brine as a slurry which may contain about 30 percent by weight of silica. According to known seeding processes, a portion of this siliceous slurry from the reactor-clarifier stage may advantageously be recirculated back upstream into the high pressure flash crystallizer, wherein the silica material in the slurry acts comprises the silica seed material.
For such reasons as aquifer replenishment and avoiding ground subsidence, the brine overflow from the reactor-clarifier, as well as steam condensate from the electric power generating facility, is usually pumped back into the ground through deep injection wells. Although a properly designed reactor-clarifier is ordinarily effective for removing most of the siliceous solids suspended in the brine, the overflow brine from the flash crystallization stage typically still contains many suspended particles which are too fine to settle out in the reactor-clarifier in a reasonable length of time. The concentration of these fine particles in the brine overflow from the reactor-clarifier is frequently sufficient to cause plugging of the injection wells at an excessive rate. Therefore, absent further treatment of the clarified brine, costly injection well rework and/or the costly drilling of new injection wells may be so frequently required that electric power production by the brine becomes uneconomical.
Normally, therefore, a clarified brine filtration stage is provided between the reactor-clarifier and the injection wells to protect the wells. When properly functioning, the brine filtration stage, which typically comprises several dual-media filters, reduces the residual suspended solids concentration in the brine to acceptable injection levels. It should, however, be appreciated that tradeoffs generally exist between the cost of increasing filter effectiveness and the cost of occasional injection well rework.
By way of illustrative example, the clarified brine overflow from a typical reactor-clarifier may, in some instances, have a residual suspended solids concentration of about 150 parts per million, with a mean particle size of between about 4 and about 5 microns. However, by effectively filtering the clarified brine, the residual suspended solids concentration may be reduced to only about 10 to 15 parts per million, with a mean particle size of between about 3 and about 4 microns. Such solids concentrations after brine filtering appear not to cause an excessive amount of damage to brine injection wells and are generally considered acceptable.
Although it is generally possible, by effective brine filtration, to achieve an acceptably reduced suspended solids concentration in clarified brine, the filtering process has itself typically been found to create new and serious problems in geothermal brine handling systems. For the filters to be effective in filtering the brine, they must, of course, remove a substantial amount of the residual, suspended solids from the brine. These removed solids necessarily accumulate in the filters and must periodically be removed in order to maintain filter effectiveness and efficiency. However, the fine siliceous particles removed by the filters from the clarified brine tend to be very "sticky" or cohesive in nature and, as a result, tend rapidly to agglomerate into sizeable clumps of material commonly referred to as "mud balls." These mud balls, which rapidly become larger and more massive than the filter media particles, cannot be easily removed from the filters by conventional backwash procedures, and even with frequent filter backwashing mud balls often still form at rates requiring the costly replacement of the filter media as frequently as every few months. Available media filters suitable for such brine filtering have, moreover, generally been found to be difficult and time consuming to repack with filter media.
Frequent filter backwashing to retard mud ball formation in the filters, and thereby prolong filter media life, has been found to cause other problems. For example, to avoid the necessity for power plant shutdown during filter backwashing, which may be required as often as every few hours in some facilities, otherwise-redundant filters must generally be provided at substantial added facility cost.
Moreover, frequent filter backwashing increases backwash disposal problems. Ordinarily the filters are backwashed, from a backwash holding tank, with filtered brine from the filters. Like the brine itself, the backwash brine must generally be reinjected into the ground as the only practical method of disposal, particularly since the material backwashed from the filters may contain potentially hazardous concentrations of such heavy metals as lead and zinc. However, because of the amount of solids swept out of the filters by the backwash brine, direct injection of the backwash brine through the brine injection wells is usually not practical. It has, therefore, been the usual practice to pond the backwash brine for a period of time during which some of the contained solids settle from the brine, and them to pump the brine, still containing some suspended solids, back upstream, for example, into the atmospheric flash vessel for recombination with the main flow of brine upstream of the reactor-clarifier stage. However, the solids suspended in the backwash-brine tend to upset the brine-solids separation process in the reactor-clarifier, thereby causing the clarified brine overflow from the reactor-clarifier to have higher than normal concentrations of suspended solids. This, in turn, overloads the filters, and accelerates the formation of mud balls in the filters, and necessitates even more frequent backwashing. Furthermore, the ponding of the backwash brine before the combining thereof with the main flow of brine causes the backwash to become more acidic, due principally to the air oxidation of ferrous ions (naturally present in the brine) to ferric ions. As a result, the corrosion of downstream brine-handling equipment is typically increased when backwash brine is ponded before injection.
These and other filtering stage problems have added significantly to the overall cost of power production from geothermal brine, and have been instrumental, along with other brine-handling problems, in making the economical production of electric power by the use of geothermal brine difficult to achieve. Consequently, improved processes for the pre-injection treatment of clarified brine are still needed.
It is, therefore, an object of the present invention to provide a process and apparatus for the secondary clarification of geothermal brine prior to the reinjection thereof into the ground.
Another object of the present invention is to provide a geothermal brine secondary clarification process and apparatus utilizing flocculants and the recirculation of brine underflow from the secondary clarification process.
Other objects, advantages and features of the present invention will become apparent to those skilled in the art from the following description, when taken in conjunction with the accompanying drawings.